1. Field of the Invention
The present invention relates generally to a silent chain having rocker joint pins, and more particularly to a silent chain including link plates articulately connected in an endless fashion by means of rocker joint pins disposed at different pitches.
2. Description of the Related Art
Silent chains are wound around at least two spaced sprockets for transmitting power between these sprockets. The silent chain includes a succession of link modules articulately connected together in an endless fashion. Each link module is formed by either a single link plate or a plurality of link plates overlapped side by side. Each link plate has a pair of bifurcated meshing portions or teeth on one longitudinal side thereof for meshing engagement with teeth of each sprocket and a pair of pin holes formed therein for receiving a pair of pivot pins, respectively.
At the onset of meshing engagement between the link plates of the silent chain and the teeth of the sprocket, respective inside or outside flanks of the teeth of the link plate are caused to interfere with the sprocket teeth. At this time, impact or collision sound occurs. Such a collision sound per se constitute a problem, but a more serious problem is that the collision sound occurs periodically.
To deal with the latter-mentioned problem, several solutions have been proposed as disclosed in Japanese Patent Publication No. HEI-3-51933 and Japanese Utility Model Publication HEI-8-1312.
A silent chain disclosed in Japanese Utility Model Publication HEI-8-1312 seeks to prevent generation of periodic collision sound by adopting link plates of more than one shape which are designed such that the interval of contact points between outside flanks of the teeth of the link plates and the sprocket teeth is variable on an every link plate basis.
According to Japanese Patent Publication No. HEI-3-51933, the silent chain uses differently profiled link plates designed such that the interval of contact points between inside flanks of the teeth of the link plates and the sprocket teeth is made variable on an every link plate basis so as to prevent generation of periodic collision sound.
The prior solutions, both relying on a varying link pitch, are still unsatisfactory due to drawbacks as discussed below.
The link plates are usually manufactured through punching processes and hence require punching molds or dies for the manufacture thereof. Especially in the case of link plates used for silent chains, a large punching force is required for die-cutting the outline of a link plate and punching pin holes in the link plate. In addition, partly because the die-cut peripheral surface of the link plate includes the outside and inside flanks adapted to be engaged with the sprocket teeth, and partly because the punched pin holes have direct effects upon the chain pitch, the peripheral surface of the link plate and inner surfaces of the pin holes must be processed with high accuracy. In a punching process, a relatively smooth sheared surface and a relatively rough broken surface contiguous with the sheared surface are formed. Since the link plate requires high dimensional accuracy and a high percent shear, the punching dies used in the manufacture of the link plate must be replaced frequently to avoid wear.
A punching die, which is highly precise and is able to withstand a heavy load, is very expensive. Thus, production of more than one type of link plates using such expensive punching dies appears to be totally uneconomical. In addition, since the silent chain has a great number of links each comprised of more than one link plate, an enormous punching-die manufacturing cost is not avoidable.
In addition, two or more types of link plates used for assembling a silent chain require a large storage space as well as an increased manpower for transportation and inventory. This will make the manufacture of the silent chain totally uneconomical.
With the foregoing drawbacks of the prior art in view, it is an object of the present invention to provide a silent chain which includes rocker joint pins arranged at different pitches along the length of the silent chain so as to reduce periodic noise generated when teeth of the silent chain come into meshing engagement with teeth of a sprocket.
Another object of the present invention is to provide a silent chain having rocker joint pins arranged at different pitches along the length of the silent chain so as to make the silent chain economical from the viewpoint of manufacture and inventory of parts of the chain.
In order to accomplish the above-mentioned objects, the present invention provides a silent chain comprising a number of link modules adjacent to each other and articulately connected together in an endless fashion by rocker joint pins. Each of the link modules is comprised of at least one link plate having a pair of spaced meshing portions on one side thereof, and a pair of pin holes spaced in the longitudinal direction of the link plate. Each of the rocker joint pins is inserted through the pin holes in the link plates of two adjacent ones of the link modules. The said rocker joint pins include a first rocker joint pin and a second rocker joint pin arranged in a random pattern along the length of the silent chain. The first rocker joint pin is comprised of a combination of a first longer pin and a first shorter pin disposed back-to-back within the same pin hole. The first longer and shorter pins each have a different first thicknesses as measured in a direction of pitch between contact points of the longer and shorter pins when the silent chain is in a straight-line state. The second rocker joint pin is comprised of a combination of a second longer pin and a second shorter pin disposed back-to-back within the same pin hole. The second longer and shorter pins each have a different second thickness as measured in the direction of pitch when the silent chain is in the straight-line state. The second thickness of the second longer or shorter pin is different from the first thickness of a corresponding one of the first longer and shorter pins.
It is preferable that the difference between the respective first thicknesses of the first longer and shorter pins and the difference between the respective second thicknesses of the second longer and shorter pins are in the range of from 0.2 to 3.0% of a reference chain pitch of the silent chain.
The silent chain, as it is about to mesh with a sprocket, takes a straight-line position. And at the onset of meshing engagement between the silent chain and the sprocket, outside teeth flanks or inside tooth flanks of the chain are caused to interfere with teeth of the sprocket. And at this moment, impact or collision sound is generated between the tooth flanks and the sprocket teeth.
It is import to note that rocker joint pins each composed of a longer and a shorter pin disposed back-to-back within one pin hole of each link plate have a pitch defined by the distance between contact points of two adjacent pairs of longer and shorter pins. At the onset of meshing engagement between the chain teeth and the sprocket teeth, the silent chain is in a straight-line state and the outside flanks or inside flanks of the chain teeth are caused to engage with the sprocket teeth. In each of the first and second rocker joint pins, a thickness of the longer pin, as measured in a direction of the pitch of contact points between the adjacent pairs of longer and shorter pins, is made different from a corresponding thickness of the shorter pin. The first rocker pin and the second rocker joint pin are arranged in a random pattern along the length of the silent chain so that the silent chain has three different pin contact-point pitches.
Since the rocker joint pins are manufactured by a multi-stage drawing process using more than two drawing dies, it is readily possible to produce a relatively thick pin (longer pin or shorter pin) and a relatively thin pin (shorter pin or longer pin) in succession. The rocker joint pin does not require a punching process and hence can be manufactured at a low cost as compared to the link plate.
In the case where two kinds of rocker joint pins are used, a total of four pins (i.e., a relatively thick longer pin, a relatively thin longer pin, a relatively thick shorter pin and a relatively thin shorter pin) must be provided. However, a longer pin and a shorter pin can be produced by cutting off from a single continuous drawn pin-like material provided that these pins have the same thickness. Accordingly, in practice, only two types of drawn pin-like materials are required. Furthermore, in the case where a relatively thick pin and a relatively thin pin are produced in succession through the multi-stage drawing process, all of the four pins can be produced from a single row material.
By using the rocker joint pins, it becomes possible to vary the contacting timing between the outside flanks or inside flanks of the chain teeth and the sprocket teeth. Variations in contact timing are effective to suppress generation of periodic noise.